This invention relates to an apparatus and method for splitting a tow of fibers. The invention has particular applicability in splitting a large tow having more than 12,000 carbon fibers.
The requirement for high production rates of carbon fibers is that the fibers should be produced in large tows containing several thousand filaments, e.g., 48,000 (48K, where xe2x80x9cKxe2x80x9d means 1,000). On the other hand, there are several applications that require the use of a thin tow having, for example, 6,000 filaments. In such applications, one should use a thin tow that is either manufactured as a thin tow or has been split into a thin tow from a large tow. However, it is extremely difficult to split a large tow, particularly of carbon fiber, into several thin tows because of problems such as fuzz formation and tow-to-tow variability of the thin tows.
Prior to this invention, various methods have been tried to spread and split large carbon fiber tows as they emanate from a carbon fiber production line. The primary problems are fiber crossovers between the small tows, fiber breakage, and creation of fiber fuzz, all of which make additional processing into useful products difficult.
Therefore, there exists a need for technology enabling the splitting of a large tow into several thin tows without fuzz formation and deterioration of the fibers.
During the course of the present invention, it was unexpectedly found that a combination of smooth grooved guide and splitter bars having crown radii and groove depths could provide a mechanical means for splitting a large carbon fiber tow (up to 320K or higher) into multiple smaller tows without fiber disorientation, fiber breakage, or creation of fiber fuzz.
This invention produces the following advantages. First, the large tow is uniformly spread without fiber crossover by passing over one or more crowned roller bars. Second, the spread tow is cleanly split using two or more eccentric splitter guide bars. Third, no air or other means of contacting or handling of the fibers is required. Fourth, the split strands are clean and contain almost no protruding fuzz or microfibers.
The apparatus and method of this invention can be used to subdivide or equally split a large tow of carbon fibers. It may be incorporated into the carbon fiber production processing line as a portion of the sizing section of the line. Or, it may be set up as a separate off-line entity to split already produced carbon fiber tows. There are a variety of reasons for reducing the tow size, one of which is to subsequently chop the smaller tows to produce a sheet-molding compound (SMC). Alternatively, the smaller tows may be individually sized with different materials as they pass through different sections of the sizing bath.
One embodiment of this invention is an apparatus for splitting a fiber tow, comprising a spreading bar comprising a crown and a circular or oval-shaped splitter bar comprising grooves around the circumference of the splitter bar, wherein each of the grooves do not circumscribe the complete circumference of the splitter bar.
Another embodiment is a method for splitting a fiber tow, comprising passing the fiber tow over a spreading bar which has a crown and passing the fiber tow over a circular or oval-shaped splitter bar comprising grooves around the circumference of the splitter bar, wherein each of the grooves do not circumscribe the complete circumference of the splitter bar.
In one embodiment, the radius of the crown (R) on the spreading bar is uniquely related to the width of the spread of the tow and/or the width of the crown (W) of a spreading bar. See FIG. 3. The ranges of the ratio R/W are about 50/1 to 3/1, preferably between 25/1 to 4/1, more preferably between 12/1 to 5/1, and most preferably about 7/1.
In another variation, the splitter bar is alternately grooved and smooth around the circumference of the bar and has precisely shaped grooves of proper depth (D) and proper center-to-center spacing (C) of the grooves on its surface. The ranges of the ratio C/D are about 50/1 to 3/1, preferably between 40/1 to 4/1, more preferably between 25/1 to 5/1, and most preferably about 16/1.
Additional advantages and other features of the present invention will be set forth in part in the description that follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. The drawings and description are to be regarded as illustrative in nature, and not as restrictive.